Solenoids are widely applied for actuation of valves and other mechanical constructions. In a conventional solenoid actuated valve, gravity or a spring is located to bias a closure member against its closed or open position, typically to define a “normally closed” valve. A plunger made from a magnetic material is arranged to move back and forth in a sleeve to operate the closure member against the force from gravity or from the spring and thereby to shift the closure member from its position of rest. The plunger is actuated magnetically via the solenoid which is energised by an electrical current. The travel of the plunger is normally limited in one direction by the closure member and in the opposite direction by a top part of a ferromagnetic material, e.g. an iron core, which is located adjacent the top of the sleeve. The solenoid normally includes a yoke which is located adjacent the top part to form a substantially uninterrupted path for the magnetic flux from the yoke into the top part to improve the force of attraction. In operation, the plunger receives considerable kinetic energy which is discharged when the plunger collides with the top part. As a result, noise, excessive wear or even breakage of the sleeve is sometimes experienced.
To alleviate the impact of the plunger against the top part, valves have been disclosed with an impact absorbing stop member located in the sleeve. In DE 100 16 600, a spring element is inserted between a top part of the sleeve and the plunger to reduce noise from the impact. In this design, the spring creates an air-gap between the top part of the sleeve and plunger. The air-gap forms a non-magnetic media which interrupts the path of the magnetic flux between the top part and the plunger and thereby reduces the attracting force. In U.S. Pat. No. 2,735,047, a plunger is energized by a coil whereby it moves rapidly upwardly. After an initial free movement, the plunger strikes a stop member in which the impact is absorbed in a sponge rubber element. Also in this design, a gap of a non-magnetic media, i.e. the rubber element, is created, and therefore also this design is inappropriate with respect to the desire of establishing a strong force of attraction between the top part and the plunger.
In EP 0 300 407, a fixed iron core is divided into an end-surface member which is contacted by a movable iron core and a body member. An elastic member is inserted between the end-surface member and the body member to suppress noise. The elastic member is inserted between the end-surface member and the body member in such a disposition that it is not across the magnetic flux. In this attempt to alleviate the impact between a movable plunger and adjacent elements without interrupting the path of the magnetic flux, the damper has, however, become inapplicable e.g. for vacuum systems or high vacuum systems wherein evacuation of the valve is required. It is therefore an object of the invention to embody a solenoid actuated valve such that the plunger can operate at a reduced noise level, has a long service life, and such that it is applicable e.g. in combination with a vacuum system.